Mevinolin, also known as Lovastatin is a potent HMG-CoA reductase inhibitor and as such is an effective antihypercholesterolemic agent. Patchett et al. (U.S. Pat. No. 4,351,844) have found that a 3,4-dihydrolovastatin analog (I) is also a potent HMG-CoA reductase inhibitor. ##STR1## Patchett, supra, describe a catalytic hydrogenation process employing tris(triphenylphosphine) chlororhodium in an aromatic solvent for the reduction of lovastatin to compound (I). This process has also been described by Kuo et al., J. Org. Chem., 48, 1991 (1983). The Patchett-Kuo process in an aromatic solvent requires a large excess (225 to 100 weight percent catalyst to lovastatin) of the expensive rhodium catalyst. This method also produces approximately 10 percent of the isomeric 3,5-dihydrolovastatin derivative (II) which required several recrystallizations and preparative HPLC for separation. When the Patchett-Kuo process is applied to synvinolin (simvastatin) 20 percent of the isomeric 3,5 dihydro simvastatin derivative is produced. ##STR2##